Astigmatism is a defect in the eye that is corrected by a lens with a non-spherical prescription. The prescription, which is usually expressed as cylinder on the patient's prescription order, causes at least a portion of the surface of the lens to have the shape of a toric segment. Hence, such lenses are called toric lenses.
While the posterior surface of a contact lens is generally spherical in configuration, where the lens is used to correct astigmatism it will have a toric configuration.
A toric corrective lens should be properly oriented, for example, angularly oriented, with respect to the eye of the wearer for optimal effectiveness. Soft contact lenses which have been designed for use to correct astigmatism are well known in the art. Generally these lenses rely on some type of ballasting method to cause the lens to ride at the proper orientation or location on the eye. An ideal lens for correcting astigmatism has good rotational orientation. That is, the intended top of the lens should be located at the top of the wearer's eye when the lens is worn.
Mold sections, that is first or female mold sections and second or male mold sections, are used together in producing a cast molded contact lens. When a toric contact lens is to be produced or manufactured by cast molding, it is important that the male and female mold sections be properly rotationally oriented, to give the desired astigmatism correction. Put another way, it is important, in order to achieve the desired astigmatism correction, that the toric zone of the lens be oriented at a specific rotational position.
In order to identify the specific rotational orientation of a toric zone of a contact lens, it is common practice to form an axis orientation mark on the lens during lens manufacture to allow the lens fitting professional to confirm the prescription of the lens, for example, while the lens is in the wearer's eye.
Previously, such an axis orientation mark was printed onto the lens, for example, using a silicone print pad. See U.S. Pat. No. 6,878,314.
More recently, the female mold section toric insert has been provided with an orientation mark which is “transferred” to the female mold section and, when the female mold section is used to produce a lens, to the lens as an axis orientation mark. The orientation mark is cut or machined onto the female toric insert by a wire electrical discharge machining computer numerically controlled machine or EDM CNC machine.
The EDM CNC machine requires an orientation mark fixture for holding the female toric insert in place during the cutting or machining process. Ideally, this orientation mark fixture should provide for the proper position, orientation and fixation of the toric insert so as to obtain accuracy and repeatability from the machining process.
However, the orientation mark fixtures that have been used in the past have not been fully satisfactory. For example, the following problems have been identified with such orientation mark fixtures. The toric inserts can be, and have been, wrongly located on the previous orientation mark fixtures, which allowed the toric inserts to be located at different positions inside the fixture leaving two degrees of freedom for the insert to move. This condition does not assure the correct position, that is the centricity, of the orientation mark with respect to the outer diameter of the insert.
In addition, with the previous orientation mark fixtures, the position of the toric insert was not fixed, since the insert was mounted on the fixture with a slide fit on different locating holes. Also, the positioning angle of the fixture locating holes was a resultant of two components: a sync plate and a dedicated fixture plate.
In summary, the previous orientation mark fixtures did not assure that the orientation mark was placed on the toric inserts in the proper position, orientation and fixation on a repeatable basis.
There is a need for new methods and apparatus to address these and other issues.